The measurement of the density of filiform particles such as tobacco or textile fibers or filaments, assembled in an elongate mass or body (e.g. a silver, a roving, a band, a wick or, a roll) is similar in a way to measuring the density of a porous material. This measurement is not easy, in particular when it is required to know the instantaneous characteristic value of the density of filiform materials in a continuous manufacturing process, for the purpose of controlling the proportioning of these materials.
This problem is encountered in particular in the textile field in measuring a textile fiber sliver intended for spinning, so as to adjust the drawing of the silver as a function of the fiber density measured over a determined section. An analogous problem is encountered in proportioning tobacco for manufacturing cigarettes.
Among the numerous ways of measuring textile fiber slivers used up to the present is, a pneumatic device whose operation is based on measuring the pressure generated by the air expelled from the sliver when the sliver is caused to pass from a channel of determined cross-section to an adjacent channel of smaller cross-section. The higher the fiber density in the sliver, the greater the increase in pressure of the expelled air for a given band-feed speed, and vice versa. The disadvantage of this device is its total dependence on the sliver-feed speed.
In another known device, the sliver is contracted to pass through a channel fed with pressurized air through a lateral conduit, the air escaping through the channel inlet and outlet openings. By measuring the pressure variation in the feed conduit, which is determined by the cross-sections of the channel inlet and outlet openings, which in turn is a function of the fiber density of the sliver passing through the openings, a signal is obtained proportional to the density. However, the fact that the air escapes through the same openings as those which are used for the passage of the sliver makes the device dependent on the material of the sliver as well as its structure. The sliver is contracted to enter the channel, and returns more or less to its initial dimensions on leaving the channel. This depends on the material of the fibers and their parallelism, which determine the degree of elasticity of the sliver. The compression and expansion of the sliver fibers at the inlet and outlet of the channel respectively influence the outflow of the air blown into the sliver, and thus falsify the signal measured in the feed conduit.
In the field of cigarette manufacture, tobacco proportioning problems are encountered which are very similar to those relating to the control of fiber density in a textile fiber sliver. At the present time, the quantity of tobacco discharged is evaluated by measuring the pressure drop through the tobacco layer, and the position of a member designed to fix the thickness of this layer is regulated. In certain cases the density of the tobacco roll is still measured with the aid of a .beta. particle source and detector. However this method of measurement is expensive and is not allowed under certain laws.